Citation: Byrnes, J. M., and D. A. Crown, Morphology, stratigraphy, and surface ...... Kreslavsky, as well as from discussions with Steve Anderson, Mark. Bulmer ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. E10, 5079, doi:10.1029/2001JE001828, 2002
Morphology, stratigraphy, and surface roughness properties of Venusian lava flow fields Jeffrey M. Byrnes1 Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
David A. Crown Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, Pennsylvania, USA Planetary Science Institute, Tucson, Arizona, USA Received 29 November 2001; revised 12 February 2002; accepted 3 April 2002; published 16 October 2002.
[1] Morphologic characteristics, flow stratigraphy, and radar backscatter properties of five
lava flow fields on Venus (Turgmam Fluctus, Zipaltonal Fluctus, Tuli Mons/Uilata Fluctus, Var Mons, and Mylitta Fluctus) were examined to understand flow field emplacement mechanisms and relationships to other surface processes. These analyses indicate that the flow fields studied developed through emplacement of numerous, thin flow units, presumably over extended periods of time. Although the Venusian fields display flow morphologies similar to those observed within terrestrial flow fields, the Venusian flow units are significantly larger and have a larger range of radar backscatter coefficients. Both simple and compound flow emplacement appear to have occurred within the flow fields. A potential correlation between flow rheology and radar brightness is suggested by differences in planform morphology, apparent flow thickness, and apparent sensitivity to topography between bright and dark flows. Distributary flow morphologies may result from tube-fed flows, and postemplacement modification by processes such as flow inflation and crustal foundering is consistent with discrete zones of increased radar brightness within individual flow lobes. Mapping of these flow fields does not indicate any simple evolutionary trend in eruptive/resurfacing style within the flow fields, or any consistent temporal sequence relative to other tectonic and volcanic INDEX TERMS: 8450 Volcanology: Planetary volcanism (5480); 6295 Planetology: Solar features. System Objects: Venus; 8429 Volcanology: Lava rheology and morphology; 5464 Planetology: Solid Surface Planets: Remote sensing; 5480 Planetology: Solid Surface Planets: Volcanism (8450); KEYWORDS: Venus, flow emplacement, lava flow field, flow morphology, lava surface roughness, radar backscatter Citation: Byrnes, J. M., and D. A. Crown, Morphology, stratigraphy, and surface roughness properties of Venusian lava flow fields, J. Geophys. Res., 107(E10), 5079, doi:10.1029/2001JE001828, 2002.
1. Introduction [2] Lava flow fields on Venus are globally distributed [Saunders et al., 1992] and are commonly associated with volcanic shields and domes, coronae, and rifts [Head et al., 1992]. The general characteristics of many flow fields have been described [e.g., Guest et al., 1992; Head et al., 1992; Roberts et al., 1992; Senske et al., 1992; Head et al., 1993; Lancaster et al., 1995; Stofan et al., 2001; Magee and Head, 2001], including gross morphologic and surface roughness properties, geologic and tectonic settings, and large-scale topographic characteristics. Previous analyses of flow margins and surface roughness characteristics of Venusian lava flows suggest the presence of flow surfaces similar to terrestrial pahoehoe and a’a [Bruno et al., 1992; Campbell
and Campbell, 1992; Campbell and Rogers, 1994; Bruno and Taylor, 1995; Pike et al., 1998]. Lava flow fields may provide constraints on Venusian resurfacing history, for which directional [Basilevsky and Head, 1995; Basilevsky and Head, 2000] and nondirectional [Guest and Stofan, 1999] models have been developed. The directional model is based on the suggestion that the surface records a progression of globally synchronous volcanic and tectonic events, whereas the nondirectional model suggests that volcanic and tectonic processes occur on regional scales and do not display a simple evolutionary trend. In order to understand styles of volcanism and the evolution of the surface on Venus, the objective of this study is to constrain emplacement processes within lava flow fields on Venus through analysis of flow lobe morphologies, stratigraphic relationships, and radar backscatter characteristics.
1 Now at Department of Space Studies, John D. Odegard School of Aerospace, University of North Dakota, Grand Forks, North Dakota, USA.
2. Background
Copyright 2002 by the American Geophysical Union. 0148-0227/02/2001JE001828
[3] Previous investigations of lava flows have examined morphologies diagnostic of emplacement style and pro-
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cesses, constraining flow field development using terrestrial examples. Walker [1972] distinguishes lava flows based on whether they are divisible into multiple flow units (compound) or are not (simple). These terms were originally applied to small-scale features observed primarily in crosssection; pahoehoe flows are considered to be almost always compound and a’a flows are commonly compound, whereas higher viscosity lavas (andesite and dacite) are more typically simple [Walker, 1972]. For a given lava composition, extrusion rate is suggested to be the primary control determining which type of flow is produced, with higher rates favoring simple flow emplacement. Guest et al. [1987] describe constraints on flow morphology that are due to limited supply and to cooling. Volume-limited flows advance until lava supply from the local source ceases. Cooling-limited flows advance until cooling at the flow front is sufficient to prohibit further movement. This may produce overflows and breakouts from the original lobe, which change its gross planform morphology. Planform morphometric properties (average thickness and ratio of maximum width to maximum length) have been related to eruption duration and underlying slope, which is suggested to be independent of flow field size [Kilburn and Lopes, 1988; Lopes and Kilburn, 1990]. Due to the complexity of subsurface transport through multiple paths in compound pahoehoe flow fields, analyses of volume- and coolinglimited flows have been primarily limited to surface-fed lava flows [e.g., Kilburn and Lopes, 1988; Pinkerton and Wilson, 1994]. [4] Morphologic analyses of Venusian lava flows rely largely on synthetic-aperture radar (SAR) data collected during the 1990 – 1994 Magellan mission. Three radar-mapping cycles resulted in the collection of single-wavelength (S-band, 12.6 cm), single-polarization (HH, horizontal transmit-horizontal receive) data at 100 m/pixel for over 98% of Venus’ surface. That data has been resampled to 75 m/pixel in the full-resolution radar mosaic data sets (FMAPs). [5] A previous survey of large (>50,000 km2) lava flow fields, termed ‘‘great flow fields,’’ on Venus focused on overall flow field morphologies, radar textures, and eruptive settings [Lancaster et al., 1995]. Lancaster et al. [1995] developed a morphologic classification of great flow fields based on 50 of the 208 large flow fields identified by Magee [1994; see also Magee and Head, 2001]. Flow fields are described as sheet, transitional, or digitate; digitate flow fields are further classified as apron, fan, or subparallel. The morphologies are attributed to differences in emplacement style, source characteristics, and local topography. The sheet flow field morphology (e.g., from Lauma Dorsa) lacks discrete sub-units and is interpreted to indicate ponded, volume-limited flow emplacement. Transitional flow fields (e.g., Neago Fluctus) are composed of multiple large sheets, creating a more digitate planform and greater length to width ratios than sheet flow fields. [6] Digitate flow fields (e.g., the lava fan on Derceto Plateau, the southeast flank of Ozza Mons, and the flow fields examined herein) are the most common morphologic type on Venus. They are composed of multiple distinct flow units that are attributed to cooling-limited flow emplacement. The apron and fan types of digitate flow fields were found to have been extruded from centralized sources (such as coronae or clusters of shields) and are distinguished by whether
the flow field completely surrounds the source vent (apron) or not (fan), reflecting the local topography. Subparallel flow fields were found to have been emplaced from less centralized sources (such as portions of rifts). It should be noted that sheet and digitate morphologies represent, respectively, simple and compound flows, although the scale of features for which these terms were defined is drastically different. This is the case because the ability to distinguish flow units within a flow field depends on the type and resolution of data available for flow mapping. Terrestrial studies of flow fields using airborne and spaceborne radar remote sensing data suggest that the ability to discriminate individual lava flow units within Venusian flow fields may not be possible for the smallest scales at which they occur, given the limitations of Magellan SAR resolution and lack of multiple wavelength and polarization data [e.g., Greeley and Martel, 1988; Gaddis et al., 1989; Plaut, 1991]. These studies indicate that features such as a’a flows and large-scale topographic elements may be distinguished using radar data; low-backscatter features, such as pahoehoe flows and cinder cones, were not reliably identified. [7] The classification scheme of Lancaster et al. [1995] is modified in a recent study of the three large Venusian volcanoes Sif, Gula, and Kunapipi Montes [Stofan et al., 2001]. Stofan et al. [2001] divide flow fields into digitate, fan, and sheet types, which are further broken into the following subtypes based on appearance in the Magellan SAR data: dark, bright-intermediate, dark-bright edges, mosaic, hummocky, or complex. The variations in flow planform morphology and surface texture, distribution of these flow field types, and shapes of the volcanoes were examined to constrain the development of each edifice. Planform morphology was related to variations in eruption style, with digitate flows resulting from high effusion rate, short duration eruptions relative to sheet flows. Surface textures evident in the radar data were interpreted to reflect local emplacement processes, such as fracturing of the surface crust during flow. This analysis indicated that the volcanoes developed through emplacement of frequent, short duration, low effusion rate summit eruptions and infrequent, large volume flank eruptions. Previous investigations had suggested that lava flows from a volcanic center display a temporal progression from long, broad sheets to shorter, digitate flows over the duration of activity [Keddie and Head, 1994, 1995]. Volcanic histories determined by Stofan et al. [2001], however, indicate that large sheets and smaller digitate flows are both emplaced throughout the duration of edifice growth, consistent with terrestrial volcanoes. [8] Previous detailed mapping of Venusian flow fields has been limited. One such study mapped flow fields in Kawelu Planitia around Sekmet Mons, with intraflow field mapping conducted for Strenia Fluctus [Zimbelman, 2000]. From that study, it was concluded that flows within Strenia Fluctus emanate from multiple sources, are interfingered, and that at least some flows are compound. No obvious pattern in flow field stratigraphy was observed.
3. Methodology [9] For the current study, lava flow fields were chosen that display numerous discrete lobes and exhibit a signifi-
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Figure 1. Locations of Venusian flow field study sites. Numbered boxes refer to subsequent figures: 2a = Turgmam Fluctus, 2b = Zipaltonal Fluctus, 2c = Tuli Mons/Uilata Fluctus complex, 2d = Var Mons flow field, 2e = Mylitta Fluctus. Base map is a Mercator projection Magellan cycle-1 FMAP, created using the NASA Planetary Data System MAP-A-PLANET website (http://pdsmaps.wr.usgs.gov/ maps.html). North is to the top of all images in subsequent figures. cant range of radar brightness, giving preference to those imaged at similar incidence angles to terrestrial airborne radar (30– 45). Five examples (Figure 1) were selected that are covered well in the Magellan cycle-1 (left-looking) FMAPs, include rift and centralized vents, and represent a diversity of geologic and tectonic settings in various regions across Venus: Turgmam Fluctus, Zipaltonal Fluctus, the Tuli Mons/Uilata Fluctus flow complex, the Var Mons
volcanic center, and Mylitta Fluctus (Figure 2). Based on stratigraphic relationships, these flow fields represent some of the most recent resurfacing (locally to regionally) within the extensive lowland plains [e.g., Crown et al., 1994; Zimbelman, 2000; Rosenberg and McGill, 2001; D. A. Crown et al., Geologic map of the Guinevere Planitia Quadrangle of Venus, map in preparation, 2002 (hereinafter referred to as Crown et al., map in preparation, 2002)],
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Figure 2. Images of Venusian flow fields (from left-looking Magellan cycle-1 FMAPs, Mercator projection), created using MAP-A-PLANET. Boxes show locations of subsequent figures, which all have sinusoidal projections. (a) Turgmam Fluctus (figure extents are 59.3– 51.9N, 214.5 – 226.6E). (b) Zipaltonal Fluctus (figure extents are 39.9– 34.3N, 246.2 – 255.2E). Intermediate-size construct (20 km diameter) is partially buried by Zipaltonal flows (see Figure 12a). (c) Flow complex composed of Uilata Fluctus and Tuli Mons flows (figure extents are 21.8 –10.5N, 310.3– 319.7E). TM marks the location of Tuli Mons; UF indicates the highest point on the edifice associated with Uilata Fluctus. (d) Var Mons flow field (figure extents are 10.2N–7.8S, 322.3 – 307.5E). (e) Mylitta Fluctus (figure extents are 47.2– 63.7S, 346.2 – 361.1E). TP marks Tarbell Patera.
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Figure 2. (continued) providing important constraints on the development of volcanic surface units. [10] Flow field base images were produced using the Integrated Software for Imagers and Spectrometers (ISIS,
available from the U.S. Geol. Surv., Flagstaff, AZ) to reproject and mosaic Magellan cycle-1 FMAP framelets. Calibration of the radar data follows Ford and Plaut [1993] and Campbell [1995]. To apply the calibration for incidence variations
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Figure 2. (continued)
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Figure 2. (continued) between each successive data row, we fit a 2nd-order polynomial for incidence angle vs. latitude for each flow field. See appendix for additional details on Magellan radar calibration. [11] Synthetic stereo anaglyphs were generated for each flow field to aid interpretation and provide qualitative
constraints on topographic effects. The anaglyphs were generated using Magellan SAR data in conjunction with the Global Topography Data Record (GTDR) [Kirk et al., 1992; D. Young, personal communication, refer to http:// www.geology.smu.edu/tectonics/young/young.html]. The
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Figure 2. (continued) GTDR represents elevation differences of 1 m, although the altimeter footprint is large (8 km). Slope information was obtained by taking representative transects along flows and calculating average slopes over a 100 km baseline (except where noted). This baseline length was
selected because it is large enough to include a significant number of data points, but small enough to show variations within each flow field. [12] Observations and mapping of the lava flow fields focused on flow morphologies, flow stratigraphy, and radar
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backscatter characteristics. Morphologic analysis was conducted to constrain lava emplacement regimes, including whether lava was emplaced in broad lobes or in channel/ levee systems and whether individual flows are simple or compound at the scale of the Magellan SAR data, as well as to constrain lava transport styles by identifying morphologic characteristics representative of surface-fed and tube-fed flows. Flow stratigraphy was examined to support morphologic observations, provide further constraints on flow emplacement, and identify, if present, evolutionary trends in eruptive style. Crosscutting relationships were also examined between flows and other spatially associated features. Radar backscatter characteristics were analyzed within and between flow units and flow fields and compared to terrestrial flow field radar data.
4. Flow Field Descriptions 4.1. Turgmam Fluctus [13] Turgmam Fluctus (58.4 – 52.7N, 215.8 – 225.1E) extends from Iris to Tikoiwuti Dorsa (Figure 2a), covering 140,000 km2 of the region between Vinmara and Ganiki Planitiae (to the west) and Virilis Tesserae (to the east). Mapping completed as part of the current analysis differs somewhat from the recently published 1:5 M geologic map of the Pandrosos Dorsa Quadrangle [Rosenberg and McGill, 2001]. Specifically, two radar-dark outcrops within the flow field that were mapped as smooth local plains material (ps) are included herein as part of the flow field, and the flow field margin is different in a few locations, although the differences in mapping do not significantly affect interpretations of flow field emplacement. The flow field was imaged at 28.7 – 31.4 incidence and displays backscatter coefficients ranging from 1.5 to 20.9 dB. Turgmam Fluctus superposes relatively radar-dark plains that exhibit numerous small volcanic constructs, fractures, and ridges. Rosenberg and McGill [2001] indicate that Turgmam Fluctus superposes their radar-dark regional plains material (prc), densely lineated material (ld), and linear belt material (bl) units. The plains deformation that produced ridges in the region primarily occurred before emplacement of the flow field; ridges are breached by lava flows and the flow field lacks features associated with shortening. Fractures are typically buried by the flows, although some fractures crosscut the uppermost surface of the flow field. The flow field emanates from the fracture system, the most recent volcanic activity having produced radar-dark flows along an 8 km portion (centered at 56.0N, 217.8E) of a NNW-SSE trending fissure set. The flow field exhibits shallow slopes (
